Tool steel for working hot metal



United States Patent 3,097,091 TQOL STEEL FOR WQRKENG HQTMETAL Teeny Hung, North Saewielrley Township, Beaver County,

Pa, assigner to United States Steel Corporation, a corporation of New .lersey No Drawing. Filed lune 9, 1961, Ser. No. 115,918 3 Claims. (Cl. 75-126) This invention relates to a precipitation-hardening steel suitable for the casting of tools for hot-working steel, particularly the guide shoes of billet-piercing mills used in making seamless steel tubes, or the like.

Piercing-mill guide shoes have been a major source of difiiculty in the production of seamless stainless-steel tubes. These tools are subjected to heavy shock and rubbing contact by the starting billet at high temperature and under high pressures, and to alternate heating by the billet and cooling by water sprays. As a result, conventional shoes of iron-chrome-nickel alloy pick up metal from the billet on their working surfaces and the projections thus formed score or scratch subsequent billets. As a matter of fact, it has for this reason been difiicult heretofore to pierce successfully billets of high-alloy chrome-nickel steels and particularly the molybdenum varieties of austenitic stainless steels, using conventional guide shoes.

I have discovered that the nickel present in conventional guide shoes is the principal cause for metal pickup from the billets. I have, furthermore, invented an austenitic chromium-manganese steel with minor amounts of aluminum and cobalt which can be precipitation-hardened at temperatures of 1450 to 1550 F., to give the hardness, toughness and resistance to wear and heatchecking necessary in tools for Working hot metal and also the desirable quality not pick up metal from the mass being worked by contact therewith, even though it is nickel-chromium stainless steel.

My improved steel has a composition Within the following ranges oft e several elements listed:

Percent Carbon From 0.20 to 0.75. Manganese From 12 to 18. Chromium From 16 to 20. Aluminum From 0.25 to 1.0. Cobalt From 0.50 to 2.50. Nitrogen From 0.10 to 0.50.

In addition to the remainder of iron and the usual impurities, nickel may be tolerated up to a maximum of 1.0%,.

and silicon to a maximum of 2.0%. Silicon is not objectionable within the limit stated, but serves no purpose. Some nickel is usually present, traceable to the scrap used in the melt, but the amount should be kept low and in no case should exceed 1.0%. Phosphorus and sulfur should be kept below 0.05% each.

The alloy, when hardened, will not be sufficiently hard if the carbon is below 0.20%. If it is higher than 0.75%, the castings tend to fire-check and crack in service. The cobalt imparts to the metal the necessary high strength at elevated temperatures encountered in hot-working operations and promotes the formation of a tough, tight scale which acts as a lubricant and increases the heat transfer from billet to shoe. The high chromium and manganese permit a liberal amount of nitrogen to be used. This aids the manganese in producing an austenitic structure and is conducive to grain refinement. I have found that a nitrogen content in excess of .50% generally leads to unsound castings because of metal gassing, even though the chromium and manganese are on the high side, so I consider .50% as the maximum nitrogen content. Silicon above 2.00% causes embrittlement and a tendency that tools made thereof will "ice toward cracking. The ability of my alloy to precipitationharden increases as the aluminum content is raised. The hardness value can be increased from the as-cast value of approximately 215 BHN to from 325 to 350' BHN with an aluminum content between 0.50/to 1.00%, by heating to a temperature between 1450 and 1550 F. for several hours. v I I The more exact preferred composition ranges of my steel are as follows:

H v p Percent Carbon Erom 0,20 to 0.75. Manganese From 12 to 15. Chromium From 16 to 19. Aluminum From 0.25 to 1.0. Cobalt From .75 to 1.75. Nitrogen From 0.15 to 0.50.

and the remainder iron plus the usual impurities but not more than 0.50% nickel and 2.0% silicon.

A typical example of the steel of my invention has the following analysis:

and the remainder iron, plus the usual impurities.

My improved alloy is readily cast by conventional foundry practice. Melting is conveniently performed in a basic-lined electric furnace. The nitrogen may be supplied by addition to the liquid metal of nitrogen-bearing manganese, for example, or ferro-chromium containing nitrogen. Aluminum is added in the ladle at the time of tapping. The ladle used should be either basic or neutral lined, since the molten metal of my analysis has a tendency to react with other types of lining.

A simple heat-treating procedure sufiices to harden the cast alloy for use as piercer guide shoes. It has been found that tools cast With the above-mentioned analysis will precipitation-harden from about 215 BHN to about 350 BHN if heated at about 1500 F. for about six hours and cooled in the air. The heat treatment as described not only increases the hardness value to make the castings serviceable as hot-mill tools but also forms a tight adherent scale which acts as a lubricant and increases the heat transfer from billet to shoe.

A BHN of 325350 in the precipitation-hardened steel is about the optimum hardness level for metal-working tools, since higher hardness results in embrittlement. Piercer-mill guide shoes of my alloy have high toughness and are able to retain hardness at the hot-working temperatures. They are thus well suited to withstand the great shock, impact and high temperatures incident to the piercing operation.

Other examples of my alloy are the following:

Exam- As PrecipilpIle C Mn Si Cr 00 Al N Cast tation- BEN hardened BHN ples used were from the foregoing Example No. 141A, first precipitation-hardened at 1500 F. for six hours and then heated for a period of eight hours at the indicated temperature.

Although this table indicates that long-time heating at the higher temperatures results in reduced hardness, it should be noted that short-time exposure to steel temperatures of 22002300 F. does not change the hardness and other physical properties appreciably. There is no phase change. During piercing, the shoes often reach a temperature of approximately 1400 F. but, since cooling water is applied to them constantly, they seldom exceed this temperature.

Although I have disclosed herein the preferred embodiment of my invention, I intend to cover as well any change or modification therein which may be made without departing from the spirit and scope of the invention.

I claim:

1. An austenitic precipitation-hardenable steel consisting of from 0.2 to 0.75% carbon, from 12 to 18% manganese, about 18% chromium, from 0.5 to 2.5% cobalt, from 0.25 to 1% aluminum, from 0.1 to 0.5% nitrogen and the balance iron plus the usual impurities including no more than 2% silicon and no more than 1% nickel, said steel hardening from about 215 BHN as cast to from 325 to 350 BI-IN on being heated from 1450 to 1550 F. for several hours.

2. An austenitic precipitation-hardenable steel consisting of about 0.4% carbon, about 12.5% manganese, about 18% chromium, about 1.25% cobalt, about 0.5% aluminum, about 0.2% nitrogen and the balance iron plus the usual impurities including not more than 0.5 nickel and not more than 2% silicon, said steel hardening from about 215 BHN as cast to from 325 to 350 BHN on being heated to from 1450 to 1550" P. for several hours.

3. A cast tool for hot-Working steel composed of steel consisting of about 0.4% carbon, about 12.5% manganese, about 18% chromium, about 1.25% cobalt, about 0.5% aluminum, about 0.2% nitrogen and the balance iron plus the usual impurities including not more than 0.5% nickel and not more than 2% silicon, said tool hardening from about 215 BHN as cast to about 350 BHN on being heated to 15 00 F. for several hours. 

1. AN AUSTENITIC PRECIPITATION-HARDENABLE STEEL CONSISTING OF FROM 0.2 TO 0.75% CARBON, FROM 12 TO 18% MANGANESE, ABOUT 18% CHROMIUM, FROM 0.5 TO 2.5% COBALT, FROM 0.25 TO 1% ALUMINUM, FROM 0.1 TO 0.5% NITROGEN AND THE BALANCE IRON PLUS THE USUAL IMPURITIES INCLUDING NO MORE THAN 2% SILICON AND NO MORE THAN 1% NICKEL, SAID STEEL HARDENING FROM ABOUT 215 BHN AS CAST TO FROM 325 TO 350 BHN ON BEING HEATED FROM 1450* TO 1550* F. FOR SEVERAL HOURS. 